如果系結屬性為空，C#WPF系結轉換器不會觸發

我目前正面臨使用 WPF MVVM 資料系結的奇怪問題。

這個想法是使用 ValueConverter，如果它不為 null，則顯示系結值，否則顯示在ConverterParameter.

Person具有以下屬性的類：Name (string), Surname (string), Country (class).

Country類具有以下屬性：CountryName (string).

當 TextBlock 系結到 a時，如果a為空，則使用 aIValueConverter 失敗。該方法甚至沒有觸發檢查系結值是否為空。Person.Country.CountryNamePerson.CountryIValueConverterConvert

同時IMultiValueConverter作業得很好，每次都觸發，無論是否為Person.Country空。

想法為什么會這樣？嘗試搜索任何 Microsoft 文章，但一無所獲。

任何幫助表示贊賞。

源 GitHub 專案可以在這里找到：https ://github.com/kevintw86/WpfBindingConverterIssue.git

系結到 IValueConverter（當 Person.Country==null 時無法正常作業）：

<TextBlock 
          Text="{Binding Person.Country.CountryName, 
          UpdateSourceTrigger=PropertyChanged, 
          Converter={StaticResource PersonConverter}, 
          ConverterParameter=- Not set -}"
          FontSize="16"/>

人物轉換器：

public class PersonConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value == null)
                return parameter;

            if (value == DependencyProperty.UnsetValue)
                return parameter;

            if (string.IsNullOrWhiteSpace(value.ToString()))
                return parameter;

            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

系結到 IMultiValueConverter（即使 Person.Country==null 也能正常作業）：

<TextBlock FontSize="16">
                <TextBlock.Text>
                        <MultiBinding Converter="{StaticResource PersonMultiConverter}"
                                  ConverterParameter="- Not set -">
                            <MultiBinding.Bindings>
                                <Binding Path="Person.Country.CountryName"
                                     UpdateSourceTrigger="PropertyChanged"/>
                            </MultiBinding.Bindings>
                        </MultiBinding>
                    </TextBlock.Text>
                </TextBlock>

人多轉換器：

public class PersonMultiConverter : IMultiValueConverter
    {
        public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
        {
            if (values == null)
                return parameter;

            if (values[0] == DependencyProperty.UnsetValue)
                return parameter;

            if (values[0] == null)
                return parameter;

            if (string.IsNullOrWhiteSpace(values[0].ToString()))
                return parameter;

            return values[0];
        }

        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
        {
            throw new NotImplementedException();
        }
    }

人物類：

public class Person : INotifyPropertyChanged
{
    private string _Name;
    public string Name
    {
        get { return _Name; }
        set 
        { 
            _Name = value;
            OnPropertyChanged();
        }
    }

    private string _Surname;
    public string Surname
    {
        get { return _Surname; }
        set 
        { 
            _Surname = value;
            OnPropertyChanged();
        }
    }

    private Country _Country;
    public Country Country
    {
        get { return _Country; }
        set 
        {
            _Country = value;
            OnPropertyChanged();
        }
    }

    #region INotifyPropertyChanged implementation

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName] string prop = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(prop));
    }

    #endregion
}

國家級：

public class Country : INotifyPropertyChanged
{
    private string _CountryName;
    public string CountryName
    {
        get { return _CountryName; }
        set 
        { 
            _CountryName = value;
            OnPropertyChanged();
        }
    }

    #region INotifyPropertyChanged implementation

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName] string prop = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(prop));
    }

    #endregion
}

主視窗視圖模型：

public class MainWindowViewModel : INotifyPropertyChanged
{
    public MainWindowViewModel()
    {
        this.Person = new Person
        {
            Name = "John",
            Surname = null,
            Country = null,
        };
    }

    #region Properties

    private Person _Person;
    public Person Person
    {
        get { return _Person; }
        set 
        { 
            _Person = value;
            OnPropertyChanged();
        }
    }

    #endregion

    #region INotifyPropertyChanged implementation

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged([CallerMemberName] string prop = "")
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(prop));
    }

    #endregion
}

uj5u.com熱心網友回復：

考慮使用TargetNullValue和FallbackValue而不是轉換器：

<TextBlock 
      Text="{Binding Person.Country.CountryName, 
      TargetNullValue='- Not set -', 
      FallbackValue='- Not set -'}"

標籤：C＃ 。网 wpf 捆绑 值转换器

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